1. Field of the Invention
The present invention relates to a zoom lens system suitable, for example, as a photographic optical system for silver-halide film cameras, digital still cameras, video cameras, etc.
2. Description of the Related Art
With the advancement of technologies for high resolution in digital still cameras, a zoom lens serving as a photographic optical system for high-resolution solid-state imaging devices requires not only a monochromatic aberration correction but also a sufficient correction in chromatic aberration. Especially, as a focal length of the zoom lens on the telephoto side becomes long with an increase in zoom ratio (an increase in variable magnification), chromatic aberration is required to be reduced in second-order spectrum as well as first-order achromatism.
Up to the present, there are a number of zoom lenses using extraordinary dispersion glass to correct the second-order spectrum of the axial chromatic aberration on the telephoto side. In addition, as an example of the zoom lens structure suitable for a high zoom ratio, there is a positive lead type zoom lens having a lens unit of positive refractive power disposed closest to the object side.
For example, zoom lenses having four lens units having positive, negative, positive and positive refractive powers, respectively, disposed in order from the object side and using extraordinary dispersion glass are disclosed in Japanese Patent No. 3097399, Japanese Laid-Open Patent Application No. 2002-62478 (corresponding to U.S. Pat. No. 6,594,087), Japanese Laid-Open Patent Application No. 8-248317, and Japanese Laid-Open Patent Application No. 2001-194590 (corresponding to U.S. Pat. No. 6,404,561).
Moreover, zoom lenses using a diffractive optical element, which is more effective in chromatic aberration correction than the extraordinary dispersion glass, are disclosed in Japanese Laid-Open Patent Application No. 9-211329 (corresponding to U.S. Pat. No. 5,872,658).
In addition, Japanese Laid-Open Patent Application No. 2001-74901 (corresponding to European Patent Application EP 1065531 A2) discloses a technology for improving the efficiency of diffraction of a diffraction grating by using a replica material in which fine ITO (indium tin oxide) particles are mixed into a resin.
In addition, Japanese Laid-Open Patent Application No. 2001-21803 discloses a zoom lens having a first lens unit having an aspherical surface formed by a resinous layer to correct monochromatic aberration.
If a diffractive optical element is used in a zoom lens, a larger effect of chromatic aberration correction is obtained than if an extraordinary dispersion glass material is used. However, the diffraction efficiency of a diffraction grating changes depending on angles of incidence of light rays. Therefore, in cases where the diffractive optical element is used in a zoom lens in which a condition of incidence of light rays changes during zooming, the location of the diffractive optical element in the zoom lens may be limited, or the diffraction efficiency may deteriorate during zooming.
Moreover, in cases where an aspherical surface is formed of an ordinary resinous material disclosed in Japanese Laid-Open Patent Application No. 2001-21803, the effect of correction of chromatic aberration cannot be obtained.